I don't think she stood in the recent election under false pretences. Her issues with the labour party, and with Starmer, who consistently refused not only to support her, but even to speak to her for two and a half years, are clear and would have been clear to her constituents. If they were paying attention, that is.
She is a conviction labour politician. If she only wanted to secure £93K a year, there were easier ways to do it. She clearly believes in what she thinks labour should stand for. Has faith in what the LP should be - and she still has this faith, according to her letter. Eventually, a last straw will happen.
The point about last straws is that there are a whole bunch of straws beforehand, and Rosie Duffield has not been shy about talking about them. To say that she shouldn't have stood in view of her misgivings is to misunderstand both her and politics. She was elected as a representative of her constituents, not a delegate. She has a duty to use her own judgement and take the decisions she thinks best in her position as a representative. She has done exactly that.
Steve Reed is my MP, and I didn't vote for him, even though he was clearly and by far the best candidate standing simply because I don't believe with Rosie Duffield that the LP can ever be on the side of the angels. I think it's a cynical authoritarian bureaucracy, despite some very good people, and that has stopped me voting for them in a general election ever since I first voted in the '70s. But I would have put this disinclination to one side and voted for Duffield, and would have applauded her resignation now. Because I trust her judgement and her courage.